In recent years, diversification of printed products has required printing on a wider variety of materials in sheets; for example, papers, synthetic papers, polymer films such as thermoplastic resin films, metallic foils, metallized sheets, etc. These printed items are printed by methods such as by offset printing, gravure, flexography, screen process printing and letterpress printing. In these printing methods, a method which uses radiation curable ink has recently become popular because radiation curable inks cure rapidly, and the printing method which uses radiation curable ink is of superior handling. Radiation curable inks are known to be useful in the printing of packaging, labels and non absorbing printing materials. Radiation curable printing inks typically contain unsaturated acrylates, polyesters, photoinitiators, and additives. In electron beam cured inks however, the photoinitiators may be omitted.
After deposition of the radiation curable ink on the printable item, the print is exposed to radiation and hardens within a fraction of a second. Printing speeds up to 300 m/min are attained during continuous printing. At present, there is a great demand for sheet-like printable items.
In printing methods, the printing sheet desirably has good sheet running properties and anti-blocking properties, producing uniform spread of the ink over the surface of the sheet, as well as antistatic properties. Besides these generally required properties, in printing methods which use radiation curable ink, the printing sheet requires in particular the property of adhering strongly to radiation cured ink.
In particular, radiation curable ink printed polymer films, intended for use as labels, for example in the bottle labelling market, should be resistant to both freezing water conditions (to allow storage of the resultant product in coolers or ice buckets) and sterilization processes for example by exposure to steam (to ensure pre-labelled bottles are fit for filling).
European patent application EP-A1-410051 discloses printing sheets comprising a support layer and a surface layer on at least one face of said support, said surface layer containing at least an acrylate based polymer and an unsaturated compound (cinnamic acid or derivatives thereof). This document does not teach anything about the possibility of use of other monomers to replace cinnamic acid.
WO-A-02/048260 discloses what are said to be improved binders, ink-receptive compositions and coated substrates containing a binder, a particulate filler and a mordant.
Preferred binders include one or more acrylic copolymers made with at least one wet abrasion resistance-enhancing monomer. The coating compositions disclosed therein are fully saturated, with ink receptivity being provided by the mordant and/or the filler.
WO-A-01/60878 discloses co-mingled polyurethane-polyvinyl ester compositions for use as coatings intended for protectivity rather than ink receptivity. This document suggests the use of a water-dispersible UV-hybrid coating which is fully cured during the manufacturing process.
US 2002/0098340 discloses a printable film comprising a substrate and at least a surface layer which covers at least one face of the substrate and which comprises a water dispersible polymer and an ethylenically unsaturated compound. However said product can be expensive and complicated to produce due to its multi-component nature. This document teaches the use of primers intermediate between the substrate and the surface layer to provide a satisfactory level of adherence.
However, the above listed materials formed in sheets, especially polymer films, do not sufficiently adhere to radiation curable ink after printing and curing, especially in these extreme conditions. Accordingly the printed and radiation cured ink is problematic in that the printed and radiation cured ink separates from the polymer film.
Improvements are required in the area of printable films, in order to achieve a product which is cost-effective, easy to manufacture, and which also has appropriate adhesion properties.